Acetylene elimination in photodissociation of neutral azulene and its cation: An ab initio and RRKM study

The ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) approach in conjunction with ab initio calculations of potential energy surfaces has been applied to investigate the photodissociation mechanism of azulene and its cation at 6.4 eV (wavelength 193 nm) upon absorption of one UV photon followed by internal conversion into the ground electronic state. Reaction pathways leading to several dissociation products have been mapped out at the Gaussian-3 level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for dissociation products have been calculated to solve the system of master equations. It is established that the major dissociation channel of neutral azulene is H atom elimination. Pentalene and phenylacetylene as heavy fragments of acetylene elimination reaction could be observed with low yields. Azulene cation dissociates preferably through the pentalene channel. C10H7+, phenylacetylene, and benzocyclobutadiene cations can also be produced with comparable concentrations. Different kinetics of the acetylene elimination reaction have been revealed in the cases of neutral and charged azulene.